Electrically scanned tracking feed

ABSTRACT

Apparatus for electrically scanning a radio frequency tracking feed beam employing a feed in which the phase center of the beam is located at the aperture. Two lobes with 180* phase reversal are provided in both azimuth and elevation by exciting the feed in the higher order modes, typically the TE20 mode for azimuth and the hybrid TE11+TM11 mode in elevation. Sequentially lobed tracking is obtained by moving the phase center to the left and the right of the focal axis of the feed for azimuth tracking and above and below the focal axis for elevation tracking.

United States Patent Leonard l. Parad Framlngbam, Mass.

Aug. 28, l969 Sept. 14, 1971 Sylvania Electric Products, Inc.

Inventor Appl. No. Filed Patented Assignee ELECTRICALLY SCANNED TRACKINGFEED 5 Claims, 25 Drawing Figs.

US. Cl 343/777, 343/854 Int. Cl l-IOlq 13/00 Field of Search 343/777,778, 779, 786, 853, 854

[56] References Cited UNITED STATES PATENTS 2,994,869 8/l96l Woodyard343/777 3,35l,944 ll/l967 Dunn et aL 343/786 3,383,688 5/1968 Renaudie343/786 3,423,756 1/1969 Foldes 343/786 Primary Examiner-Eli LiebermanAttorney-Robert T. Orner ABSTRACT: Apparatus for electrically scanning aradio frequency tracking feed beam employing a feed in which the phasecenter of the beam is located at the aperture. Two lobes with 180 phasereversal are provided in both azimuth and elevation by exciting the feedin the higher order modes, typically the TE, mode for azimuth and thehybrid TE +TM mode in elevation. Sequentially lobed tracking is obtainedby moving the phase center to the left and the right of the focal axisof the feed for azimuth tracking and above and below the focal axis forelevation tracking.

all L I ll] PATENIED SEP I 4 I971 3. 6 O5 1 0O SHEET 2 [IF 4 DIRECTIONOF DISTRIBUTION OF ELECTRIC FIELD ELECTRIC FIELD TE) II I I I 4A. b E

I zc IIII I I I7 I I V TE11+ TM" 46 III III IIIIIII IIIIIIIII IIIIIIIITE -I-TE III III I I 10" 2o I II I I M I TE (I-E" +TM") IIII IIIIIIIIIIIII 4. I.\'\'III\"I'()I\ LEONARD I. PARAD SHEET 3 BF 4 E of TE E ofT5 TO LOGIC CIRCUIT V l\\\n DRIVER E of TE +TE Fig. 5.

e0 FLIP-FLOP FLIP-FLOP OSCILLATOR A D Fig. 6'.

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J EXCLUSIVE v WITH COMPLEMENT L G DRIVER PROBE DRIVER DRIVER LEONARD I.PARAD HYQ'DMZJW PATENTEUSEPIMQYI 3.605.100

SHEET Q UF 4 VOLTAGE H I I OSCILLATOR 60 7A.

H FF 62 73 H FF 62 K FF 64 L H a 7 D. DRIvER 74 L FF 64 K a Y E. DRIvER76 EX. OR 66 G H 8 TH DRIVER 72 L Ex. OR 66 H H a 7G. DRIVER 70 PROBETABLE 7H. NUMBER CONDITION OF PROBE 34 OPENED sHORTED sHORTED OPENEDOPENED 35 sHORTED sHORTED OPENED OPENED SHORTED 3e OPENED OPENED sHORTEDsHORTED OPENED 37 sHoRTED OPENED OPENED sHoRTED sHORTED POSITION OFPHAsE OENTER RIGHT DOWN LEFT UP RIGHT to t1 t2 t3 t4 5 I ZII'\'\'I'.'.\"I()I\' LEONARD I. PARAD ATTORNEY EIJEC'I'RICALLY SCANNEI)TRACKING FEED BACKGROUND OF THE INVENTION This invention relates toantenna systems and in particular to a: :nna feed assemblies useful, forexample, in radar tracking system employing sequential lobing.

Tracking or direction finding by sequential lobing is performed bysequentially comparing the signals received via antenna patterns atvarious positions about its focal axis. The most common techniques forsequential lobing employ beam switching or conical scan. When an RFsignal appears within each of two beams at an angle, 6, with input tothe boresight axis of an antenna, a voltage of amplitude E,(6) isreceived by the upper beam and a voltage amplitude (6) is received bythe lower beam.By switching an antenna beam alternately between thesetwo positions, the two amplitudes of the received voltages may becompared. If the source is on the boresight axis, the voltages areequal. If the source is above the boresight axis, E,(6) is greater thanE and if the source is below the boresight axis, E 9) is greater than ETwo additional switching positions are needed to obtain information inthe orthogonal coordinate. Thus, a two dimensional sequentially lobingantenna might consist of a cluster of four feed horns illuminating asingle reflector, arranged so that the right-left and up-down sectorsare covered by successive antenna positions.

Another technique for obtaining tracking information is to continuouslyrotate an offset antenna beam rather than to discontinuously step thebeam among four discrete positions. Apparatus for obtaining acontinuously rotating beam includes a parabolic reflector with an offsetrear feed rotated about the axis of the reflector. Or, if the antenna issmall, the reflector itself may be rotated. In either case, there ismechanical movement between the reflector and the feed to achieverotation of the beam in space. For large antennas, the problemsassociated with relative movement between the reflector and feed areincreased.

It is, therefore, an object of this invention to provide a rotatingantenna field pattern from a single horn and reflector, both of whichare stationary with respect to each'other.

SUMMARY OF THE INVENTION In accordance with the present invention, anantenna feed apparatus employs a first means for propagating a firstelectromagnetic field configuration in response to an input signal atone end. Connected to the other end of the first means for propagatingand coupled to a means for generating second and third fieldconfigurations is a second means for propagating which propagates notonly the first field configuration but also second and third fieldconfigurations. A means operable to sequentially add and subtract thefirst field configuration to the. second and third field configurationsin a predetermined manner is connected to the second means forpropagating such that the phase center of the resultant field is rotatedat the output end of the second means. When the feed apparatus isemployed in conjunction with a focusing wave translation means such as areflector system, a far field pattern is generated which is switchedabout the focal axis of feed and reflector combination.

DESCRIPTION OF THE DRAWINGS The construction and operation of theinvention will be more fully understood from the following detaileddescription taken in conjunction with the accompanying drawings inwhich:

FIG. IA is a representation of an antenna feed apparatus according tothe invention in combination with a reflector;

FIG. IB is a resultant far field pattern of the apparatus of FIG. IA;

FIGS. 2A, 2B and 2C are top, side and end views, respectively, of oneembodiment of an antenna feed apparatus according to the presentinvention;

FIGS. 3A-3D and FIGS. 4A-4G are waveshapes and electric fieldconfigurations useful in explaining the theory of operation in theapparatus of FIG. 1;

FIG. 5 is a view of a diode and probe connection employed in theapparatus of FIG. 1;

FIG. 6 is a schematic diagram of a logic circuit employed to scan thephase center of the antenna feed apparatus of FIG. 2; and

FIGS. 7A-7G are waveshapes useful in explaining the operation of thelogic circuit of FIG. 6.

DETAILED DESCRIPTION OF THE INVENTION Referring now to FIGS. 1A and KB,a horn assembly 10 is employed in an electromagnetic wave translationarrangement with a focusing wave translation means such as a parabolicreflector 12, to form a far field antenna pattern in response to a radiofrequency signal at the input end 11 of the born 10. When the electricalphase center of the horn I0 is shifted to one side 14 of the horn 10, afirst antenna field pattern 20 results. Similarly, when the electricalphase center of the horn 10 is shifted to the other side 18, a secondantenna field pattern I6 is generated. By sequentially shifting theelectrical center from one side of the horn to the other, sequentiallobes (antenna field patterns) are generated about a common focal axis.

While two sequential lobes are shown, it is to be understood that asecond set of lobes can be generated by moving the electrical phasecenter of the horn 10 in a direction orthogonal to the directiondictated by sides 14 and 18.

An embodiment of a horn 10 according to the present invention is shownin top, side and end views in FIGS. 2A, 2B and 2C, respectively. Thehorn 10 includes a first section of transmission line, for example, asection of rectangular waveguide 30 having predetermined innerdimensions a, b to support a single electromagnetic field configurationor mode such as the TE mode. Connected to the first section of waveguide30 is a second section of waveguide 32 of predetermined inner dimensionsa', b sufficient to support not only the first mode but also second andthird modes such as the TE and the hybrid TE +TM modes. By properphasing of the TE mode with the TE mode, the horn phase center can beshifted in one plane and similarly by proper phasing of the TE mode withthe hybrid TE +TE mode to the phase center can be shifted in anorthogonal plane as will be explained hereinbelow.

Coupled to the second section of waveguide 32 is means for generatingthe second and third modes in combination with the first mode. The meansincludes a plurality of probes 34, 35, 36 and 37, respectively,projecting into the second section of waveguide 32 but electricallyisolated from its by the insulators 39. The depth and position of theprobes and their effect on the antenna lobes relative to the focal axis22 will be discussed hereinafter.

In general, a section of waveguide has a particular cutoff wavelength.When the frequency of the signal is high enough to permit thetransmission of more than one mode, the resultant field is the sum ofthe fields of the individual mode fields propagating in the guide. Ifthe fields of one mode are stronger than those of the others, this modepredominates.

For example, if a rectangular waveguide, as illustrated in cross sectionin FIG. 3A, is excited in the TE mode, the electric field variationacross the guide is sinusoidal as shown in FIG. 3B. The TE mode can belaunched in the waveguide by any of the well-known techniques, severalof which are given in the Reference Data for Radio Engineers, FourthEdition, by International Telephone and Telegraph Corporation. Assumethat the b dimension of the guide is less than a half wavelength so thatno TE mode can be supported and that the a dimension exceeds onewavelength so that the "IE mode can be transmitted by the guide. Theelectric field distribution of the TE mode is shown in FIG. 3C.

If only the TE mode is excited, no TE mode will be transmitted. If,however, an asymmetrically located probe projects into and iselectrically connected to the guide, as shown in FIG. 3A, the totalelectric field configuration will become asymmetrical, as shown in FIG.3A, with a resultant field distribution, as shown in FIG. 3D. When theswitch is closed shorting the probe to the waveguide, the probe is areceiving antenna that extracts energy from the incident TE, mode waveand reradiates it so as to excite the TB mode. When the switch 8 isopen, the probe merely blocks a small amount of energy.

Note that in FIG. 3D, the phase center of the electric field has beenshifted to one side of the waveguide by shorting the probe to thewaveguide. When employed in conjunction with a reflector 12, as shown inFIG. 1A, the resultant far field antenna pattern or lobe would bepositioned to one side of the focal axis 22.

To shift the antenna pattern about a focal axis in two orthogonalplanes, for example, azimuth and elevation planes, two particular modesare required in conjunction with the basic TE mode of FIG. 4A. Forazimuth tracking, a mode which radiates two lobes with 180 phasereversal is required. The TE mode illustrated in FIG. 4B satisfies thisrequirement. Similarly, the hybrid TE -l-TM mode shown in FIG. 4Cprovides the requisite lobes for elevation information.

A sequentially lobed tracking antenna is obtained if the phase center ofthe feed is moved to the left and right of the focal axis for azimuthtracking and above and below the focal axis for elevation. The phasecenter can be shifted in the azimuth plane (left and right) by combiningin phase and out of phase the TE and TE modes. By adding the TE and theTE modes, the phase center is shifted to the left as shown in FIG. 40.Similarly, by subtracting (adding out of phase) the TB mode from the TEmode, the phase center is shifted to the right as illustrated by FIG.4E.

Elevation patterns, above and below the focal axis, are obtained bycombining the hybrid TE +TM mode in and out of phase with the TE mode asshown in FIGS. 4F and 4G respectively.

Apparatus for generating and phasing the higher order modes forelevation will be discussed in detail, and the extension to the azimuthplane will be apparent. The smaller section of waveguide 30 of the hornl propagates only the dominant TE mode. At the junction of the small andlarge sections of waveguide, a number of higher order modes are excited.However, the dimensions a and b are chosen in accordance with well-knownprinciples to suppress all symmetrical higher order modes generated atthe symmetrical junction. Thus, if the larger section 32 is symmetrical,only the TE mode will exist in it.

The symmetry of the large section 32 van be eliminated by the additionof the probes 34-37 as shown in FIG. 2. All four probes extend the samedistance into the large section 32. By shorting two of the probes, forexample, probes 36 and 37, to the structure, an asymmetry is createdwhich converts some of the TE mode energy into TE and TM mode energy.The phase relationship between the TE and the resultant TE -l-TMmode canbe controlled in several ways, for example, by physically orelectrically changing the length L of the section 32 or by selectiveshorting and opening of the appropriate probes.

In employing the latter technique of shifting the phase center about thefocal axis, the probes 34-37 are electrically shorted to or isolatedfrom the horn structure in accordance with the following table:

One means for shorting and opening the electrical connection of a probeto and from the horn structure is shown in FIG. 5 and includes a diode70 having one end connected to the horn 10 and the other end connectedto one of the probes, for example, probe 34. The insulator 39 normallyisolates the probe 34 from the horn structure. By applying theappropriate biasing to the diode 70 from a logic circuit to be discussedin detail hereinafter, the probe can be isolated from or shorted to thehorn 10.

In a sequential-lobing tracking system, an antenna pattern issequentially moved in a predetermined pattern, for example, a clockwisepattern among four quadrants, two in azimuth and two in elevation, todetermine positional information of a target. One embodiment of a logiccircuit for programming the opening and closing of the diodes to cause aclockwise rotation of phase center (and therefore a counterclockwiserotation of the antenna pattern) is shown in FIG. 6 and includes asquare wave oscillator 60, the output of which is connected to the inputterminal A of a first flip-flop 62. The first output terminal B offlip-flop 62 is connected to a first input terminal J of an exclusive ORwith complement circuit 66 such as a Sylvania SG- integrated circuit.The second output terminal C of flip-flop 62 is connected to the inputterminal D of a second flip-flop 64 and to a second input terminal K ofthe exclusive- OR with complement circuit 66. The first output E of thesecond flip-flop 64 is connected to a third terminal of the exclusive ORwith complement circuit 66 and to a first driver circuit 74. The secondoutput terminal F of the second flipflop 64 is connected to the fourthinput terminal L OF THE EXCLUSIVE OR with complement circuit 66 and to asecond driver circuit 76. The first and second output terminals H and Gare connected to respective driver circuits 72 and 74, and the outputterminals of the drivers 70, 72, 74 and 76 are connected to the diodesassociated wit respective probes 35, 36, 34 and 37.

The waveshape of FIG. 7 will be employed to explain the operation of thelogic circuit of FIG. 6. In response to the oscillator 60 output signal,shown in FIG. 7A, the first flip-flop 62 produces a first output signaland its complement at terminals B and C respectively. These signals areshown in FIGS. 78 and 7C and are at a frequency equal to half that ofthe oscillator 60. Similarly, the second flip-flop 64 produces a firstoutput signal and its complement at its terminals E and F in response toan input signal at its input terminal D from the first flip-flop 62. Theoutput signals, as shown in FIGS. 7D and 7E, are equal in frequency toone-quarter of the frequency of the output signal from the oscillator60.

The two flip-flops 62 and 64 are employed to count the oscillator signaldown to produce square waves at one-half and one-quarter the oscillatorfrequency. The output signals of the flip-flops 62 and 64 are combinedin the exclusive OR with complement circuit 66 to produce a second setof signals, as shown in FIGS. 7F and 7G, at one-quarter the frequency ofthe oscillator signal. The E and F output signals of the secondflip-flop 64 and the H and G output signals of the exclusive OR withcomplement circuit 66 are square waves of one-quarter the oscillatorfrequency but with four different phase relationships. Assume the diodesare forward biased (shorted) when their respective drivers are in thelow voltage, L, output state and the diodes are reverse biased (opened)when their respective drivers are in the high voltage, l-l, outputstate.

Shown in FIG. 7H is a table indicating the operating condition of thediodes (and therefore the condition of the probes with respect to horn10) for one cycle of clockwise rotation of the phase center about thefocal axis of the horn 10. For example, to position the phase center tothe left of the focal axis, the diodes associated with probes 34 and 36are forward biased shorting the probes to the horn and conversely thediodes associated with probes 35 and 37 are reverse biased isolating theprobes from the horn. The speed of rotation can be increased ordecreased by increasing or decreasing respectively the frequency of theoscillator 60 or the sequence of phase center movement can be changed byconnecting the drivers to the appropriate probes.

Referring again to FIG. 2, the exact position of each of the probes34-37 relative to the vertical and horizontal centerlines of the horn l0and the junction of waveguide sections 30 and 32 is determined by thedesired antenna characteristics. The amount of TE energy converted intoTE and TM, ,+TE H modes is proportional to the depth, u. of the probesinto the large section 32. The ratio of azimuth to elevation shift isdetermined by the distance x relative to the centerline 33. The shorterthe .r distance is, the greater the elevation shift and the less theazimuth shift. The distance .r relative to the centerline is, therefore,a compromise between competing requirements, which requirements arebalanced by judicious placement of the probes in accordance with thedesired shift in each plane.

The location of the probes relative to the junction of sections 30 and32, as indicated by the y dimension in FIG. 2A, is based upon atrade-off of the competing modes, TE and TM -l-TEmodes. Ideally, theprobes should be located at the first maximum of the electric field inthe waveguide section 32. However, since the maximums for the TE and theTM,,+TEmodes do not occur at the same point, the y distance must beoptimized to obtain a desired amount of both modes consistent with thedesired phase center shift.

The amount of TE energy converted into the higher order modes is also afunction of the diameter D of the probes. In addition t0 the amount ofenergy conversion, the diameter D of the probes governs the O of thefeed apparatus. The larger the diameter D, the lower the Q and thebroader the bandwidth of the mode excitation mechanism.

Also included in the second waveguide section 32 of the horn is aconducting septum 31 to reduce residual crosspolarized energy of thehybrid TE,,+TM mode.

For example, an antenna feed apparatus according to the presentinvention was constructed to operate at 14.4 to 15.2 GHz. The dimensionsof the feed as defined in FIGS. 2A-2C are as follows:

a=0.549 inches b=0.256 inches a'=0.940 inches b'=0.676 inches x=0.l25inches u=0. 147 inches \=0.3 l 2 inches L=2.30 inches D=0.0588 inchesZ=0.661 inches The characteristics of this feed apparatus in azimuth (A)including the half power beam width (l-IPBW) of both the primary patterndata (data from the horn l0 alone) and the secondary pattern data (datafrom the combination of the horn and a reflector assembly) are includedin respective tables II and 111.

TABLE II PRIMARY PATTERN DATA HPBW (Degrees) SECONDARY PATTERN DATA(Near Fie1d)f/D=0.5

HPBW (Degrees) j E Plane H Plane Table 111 Continued An antenna feedapparatus has been described in which the phase center can beelectrically positioned such that when the feed apparatus is employedwith a reflector a resulting antenna pattern can be shifted in spacewithout physical movement of the feed apparatus relative to thereflector.

What is claimed is:

1. An antenna feed apparatus comprising:

first transmission line means having first and second ends forpropagating a first electromagnetic field configuration in response to asignal at said first end;

means for generating second and third electromagnetic fieldconfigurations;

second transmission line means including a section of waveguide forpropagating said first, second and third electromagnetic fieldconfigurations having a first end coupled to the second end of saidfirst transmission line means;

said means for generating including a plurality of fixed probes eachhaving one end protruding into said section of waveguide;

means coupled to the other end of said plurality of fixed probesoperable to add and subtract in a predetermined order said firstelectromagnetic field configuration with each of said second and thirdelectromagnetic field configurations propagated in said second means tothereby form a resultant electromagnetic field configuration, the phasecenter of which moves in a predetermined pattern at the second endofsaid section of waveguide.

2. An antenna feed apparatus according to claim I wherein:

said first, second and third electromagnetic field configurations arethe TE TE and the TE +TM modes, respectively; and

said section of waveguide includes a length of rectangular waveguidehaving predetermined dimensions, to propagate said TE TE and TE +TMmodes.

3. An antenna feed apparatus according to claim 2 wherein said means forgenerating said second and third electromagnetic field configurationincludes:

a plurality of probes having one end protruding into said length ofrectangular waveguide and being mounted on the larger sides of saidlength of rectangular waveguide at predetermined distances from thecenter and first end of said length of rectangular waveguide; and

means for electrically attaching the other end of said probes to saidlength of rectangular waveguide in a predetermined pattern to therebygenerate said second and third electromagnetic field configurations.

4. An antenna feed apparatus according to claim 3 wherein means operableto add and subtract said first electromagnetic field configuration witheach of said second and third electromagnetic field configurationsincludes:

a plurality of diodes each having one end connected to the other end ofsaid plurality of probes and the other end connected to said length ofrectangular waveguide; and

means connected to said plurality of diodes for selectively forward andreverse biasing certain cones of said plurality of diodes in apredetermined sequence to thereby short the respective probes to saidlength of rectangular waveguide causing the field configurations to addand subtract in a predetermined pattern.

5. An antenna feed apparatus according to claim 4 wherein:

said plurality of probes includes first, second, third and fourthprobes, said first and second probes being mounted on one of the largersides of said length of rectangular waveguide and said third and fourthprobes being mounted diametrically opposite said first and second probesrespectively on the other of the larger sides of said length ofrectangular waveguide; and

the predetermined pattern produced by said means for electricallyattaching the other end of the first, second, third and fourth probes tosaid length of rectangular waveguide is in accordance with the followingtable where t through 1 are sequential time intervals to thereby causerotation of the phase center of the resultant electromagnetic fieldconfiguration in a clockwise pattern at the output end of said sectionof rectangular waveguide.

1. An antenna feed apparatus comprising: first transmission line meanshaving first and second ends for propagating a first electromagneticfield configuration in response to a signal at said first end; means forgenerating second and third electromagnetic field configurations; secondtransmission line means including a section of waveguide for propagatingsaid first, second and third electromagnetic field configurations havinga first end coupled to the second end of said first transmission linemeans; said means for generating including a plurality of fixed probeseach having one end protruding into said section of waveguide; meanscoupled to the other end of said plurality of fixed probes operable toadd and subtract in a predetermined order said first electromagneticfield configuration with each of said second and third electromagneticfield configurations propagated in said second means to thereby form aresultant electromagnetic field configuration, the phase center of whichmoves in a predetermined pattern at the second end of said section ofwaveguide.
 2. An antenna feed apparatus according to claim 1 wherein:said first, second and third electromagnetic field configurations arethe TE10, TE20 and the TE11+TM11 modes, respectively; and said sectionof waveguide includes a length of rectangular waveguide havingpredetermined dimensions, to propagate said TE10, TE20 and TE11+TM11modes.
 3. An antenna feed apparatus according to claim 2 wherein saidmeans for generating said second and third electromagnetic fieldconFiguration includes: a plurality of probes having one end protrudinginto said length of rectangular waveguide and being mounted on thelarger sides of said length of rectangular waveguide at predetermineddistances from the center and first end of said length of rectangularwaveguide; and means for electrically attaching the other end of saidprobes to said length of rectangular waveguide in a predeterminedpattern to thereby generate said second and third electromagnetic fieldconfigurations.
 4. An antenna feed apparatus according to claim 3wherein means operable to add and subtract said first electromagneticfield configuration with each of said second and third electromagneticfield configurations includes: a plurality of diodes each having one endconnected to the other end of said plurality of probes and the other endconnected to said length of rectangular waveguide; and means connectedto said plurality of diodes for selectively forward and reverse biasingcertain cones of said plurality of diodes in a predetermined sequence tothereby short the respective probes to said length of rectangularwaveguide causing the field configurations to add and subtract in apredetermined pattern.
 5. An antenna feed apparatus according to claim 4wherein: said plurality of probes includes first, second, third andfourth probes, said first and second probes being mounted on one of thelarger sides of said length of rectangular waveguide and said third andfourth probes being mounted diametrically opposite said first and secondprobes respectively on the other of the larger sides of said length ofrectangular waveguide; and the predetermined pattern produced by saidmeans for electrically attaching the other end of the first, second,third and fourth probes to said length of rectangular waveguide is inaccordance with the following table t1 t2 t3 t4 first probe shortedshorted opened opened second probe opened shorted shorted opened thirdprobe shorted opened opened shorted fourth probe opened opened shortedshorted where t1 through t4 are sequential time intervals to therebycause rotation of the phase center of the resultant electromagneticfield configuration in a clockwise pattern at the output end of saidsection of rectangular waveguide.